With the scaling of integrated circuits, metal-oxide-semiconductor (MOS) devices are becoming increasingly smaller. The junction depths of the MOS devices are also reduced accordingly. This reduction causes technical difficulties during the formation processes. For example, small MOS devices demand high doping concentrations in source and drain regions in order to reduce sheet resistance in the source and drain regions. Controlling implantation depth for forming shallow source and drain junctions while at the same time maintaining high doping concentration is difficult.
Conventionally, to achieve high doping concentration in the source and drain regions, arsenic is doped heavily into source and drain regions. Arsenic has relatively low diffusion length, and thus can be implanted to a high concentration without significantly affecting short channel characteristics and junction abruptness. However, arsenic atoms are heavy with an atomic weight of about 75. The implantation of arsenic thus introduces greater degree of defects, such as piping, than introducing other n-type impurities, such as phosphorous.
In small-scale MOS devices, for example, MOS devices formed using 65 nm technology or below, the junctions are shallow, and hence the pipes caused by arsenic implantation may extend to the junctions between source/drain regions and semiconductor substrates. Accordingly, when silicide regions are formed on source and drain regions, silicide spikes may be formed along the pipes, and thus shorting the silicide regions and the semiconductor substrates. This results in a significant increase in leakage currents, and even device failure.
A new MOS device structure and manufacturing methods for solving the above-discussed problems are thus needed.